QUESTION: What's the best way to clean oil and grease from a concrete floor?

ANSWER: If oil has been freshly spilled onto a concrete surface, blot up as much as you can, then cover the spot with one of the following: powdered calcium carbonate, hydrated lime, talc or fuller's earth. Portland cement can also be used. Let powder stand for 24 hours and then scrape it off.If oil has penetrated into the concrete, scrape off whatever remains on the surface with a putty knife and then cover the stain with a stiff poultice made from one of the powdered substances above combined with a solution of 1 part trisodium phosphate and 6 parts water. Let the poultice stand 24 hours, then scrape it off. Scrub the surface with clean water.

Grease is easier to remove than oil. If scraping and scrubbing don't work, use a poultice made from benzene, naphtha or trichloroethane and an inert powder. Allow it to stand 24 hours, then scrape the surface clean.

QUESTION: I want to buy a fire extinguisher for my home, but I don't know which type to get. What do you recommend?

ANSWER: The wrong type of fire extinguisher could do more harm than good. It must suit the type of fire that's burning.

There are three types of fires. Class A fires are those that involve ordinary combustibles such as wood, paper, cloth, rubber. Home fires of this type often start in the living room or bedroom. Class B fires involve cooking oils, grease, gasoline, paint thinners and other flammable liquids. These fires generally break out in kitchens and garages. Class C fires are electrical fires and are usually the result of faulty wiring, overloaded circuits or faulty electrical appliances.

On fire extinguishers, these categories are designated by the letters A, B and C within a triangle, square and circle, respectively. Class B-, or BC-rated extinguishers are not effective on Class A fires. Also, water, which is effective in putting out a Class A fire will cause a Class B fire to spread and can cause a severe shock in a Class C fire. Once a fire in the home spreads, it can quickly include all three categories. Therefore, your best choice is a fire extinguisher rated for all three classes of fire.

Fire extinguishers are available at hardware stores and home centers. When you buy one, check to see that it's listed by Underwriters Laboratories Inc. and displays the A, B and C designations. Also, note the numbers in front of the A and B designations. These refer to the size of fire that the extinguisher can generally handle. The numbers are not absolute figures but are relative terms for comparing different units. For example, an extinguisher with a rating of 2A:40B:C will handle a Class A fire twice as large, and a Class B fire four times as large, as a unit rated 1A:10B:C.

Note that there are no size ratings for Class C fires. The C designation only means that the chemical inside will not conduct electricity.

QUESTION: While installing a new roof, the contractor recommended two large spinners that turn in the wind for ventilating the attic. My question is whether to cover them in the winter or leave them open.

ANSWER: The only reason we know of for covering the turbine vents is if you live in an area where the winter snowfall is great enough to cover the vents. In this case, when the snow melts, water will leak into the attic through the openings between the vent vanes.

If the attic's ventilation is dependent on the turbine's free air opening, you must provide alternative ventilation during the winter when the turbines are covered. The unobstructed vent opening should be 1-300th of the attic floor area when there is a vapor barrier on the underside of the insulation, and 1-150th of the attic floor when there is no vapor barrier.

For further information on any home problem, write to: Popular Mechanics, Readers Service Bureau, 224 West 57th St., New York, NY 10019. Phone: 212-649-3127.